This invention relates to a single tube Coriolis flowmeter and in particular, to a method and apparatus for a Coriolis flowmeter having a balance bar that enhances the accuracy of the flowmeter by increasing the flow sensitivity, reducing the meter shaking with flow, and making the meter flow sensitivity independent of material density.
Single tube Coriolis flowmeters are desirable because they eliminate the expense and the problems of flow splitting manifolds of dual tube Coriolis flowmeters. Single tube Coriolis flowmeters have the disadvantage that their flow sensitivity is lower than that of dual tube Coriolis flowmeters. The flow sensitivity is lower for two reasons. The first is that a single tube flowmeter must have a larger diameter flow tube for a given flow rate. This makes it stiffer in bending and less responsive to Coriolis forces. The second reason has to do with the details of how the mass flow rate is determined and the fact that the balance bar does not experience Coriolis force.
In dual tube Coriolis flowmeters, the flow tubes are vibrated out of phase with each other. The dual flow tubes counterbalance each other to create a dynamically balanced structure. Velocity sensors (pick offs) are located at two locations along the flow tubes to sense the relative velocity between the flow tubes. The pick offs are usually located equal distances upstream and downstream from the tubes"" midpoints. Each pickoff consists of a magnet fastened to one flow tube and a coil fastened to the other. The relative motion of the coil through the magnetic field produces a voltage. The sinusoidal motion of the vibrating flow tubes produces a sinusoidal voltage in each pickoff. When there is no material flow, the voltages from the two pick offs are in-phase with each other. With material flow, the vibrating tubes are distorted by the Coriolis force of the moving material to cause a phase difference between the two pickoff voltages. The mass flow rate is proportional to this phase difference. It is important to note that both flow tubes are distorted equally (for an equal division of flow) and each flow tube has the same phase shift as the other at corresponding locations. The upstream pickoff magnet velocity has the same phase as the upstream coil velocity and both have the same phase as the voltage generated by the magnet-coil pickoff pair. The downstream pickoff has a different phase than the upstream pickoff; but the phase of the magnet velocity, the coil velocity, and the output voltage of the downstream pickoff are equal to each other.
In most single tube flowmeters, the vibrating flow tube is counterbalanced by a balance bar rather than another flow tube. The exception is Coriolis flowmeters in which a flow tube is driven in the second bending mode with the flow tube being counterbalanced by a pendulum attached to the flow tube. EP 0 908 705 A2 discloses a Coriolis flowmeter that uses a single pendulum as a counterbalance. A Coriolis flowmeter using a pair of pendulums affixed to a flow tube as counterbalances is shown in Japanese document PHD11-44564.
In Coriolis flowmeters driven in the first bending mode and counterbalanced by a balance bar, the pickoff magnets (or coils) are mounted to the balance bar as though it were the second flow tube described above. However, since material does not flow through the balance bar, it does not experience any Coriolis force. The balance bar does experience some torque from the Coriolis induced deflection of the flow tube, but the resultant small deflection of the balance bar results in a small phase shift at each pickoff location on the balance bar that is of the opposite sign as the phase shift at the pickoff locations on the flow tube. The pick offs sense the relative velocity between the phase shifted flow tube and the oppositely phase shifted balance bar.
To determine the phase of the output signal, the flow tube and balance bar velocities at each pickoff are represented by velocity vectors having phase angle and amplitude. The relative velocity (and voltage out of each pickoff) can be determined by adding the two velocity vectors. The flow tube velocity vector has a phase shift due to material flow. The balance bar velocity vector has a small phase shift of opposite sign. Adding these vectors gives the net phase shift with flow of the pickoff. The net phase shift and output voltage of each pickoff is reduced by the oppositely phase shifted balance bar. This net phase shift reduction equates to a reduction in the flow sensitivity of the flowmeter.
All straight tube Coriolis flowmeters have a problem in that the flow tube geometry is inherently stiff and cannot be bent or deflected along its longitudinal axis with the same ease as can a conventional u-tube flowmeter. Single straight tube meters have an additional problem in that the single tube diameter must be increased over the diameter of the dual tubes in order to pass the same flow with the same pressure drop through the meter. The increased tube diameter stiffens the flow tube further. As a result, the flow tube of a single straight tube meter is inherently insensitive to Coriolis forces because of its stiffness. The reduction in flowmeter sensitivity due to the opposite phase shift of the balance bar combined with the reduction in sensitivity due to the larger (single) flow tube diameter results in a combined flowmeter sensitivity so low as to impair the accuracy and commercial acceptance of single tube flowmeters for some applications.
It is a further problem that existing single tube Coriolis flowmeters use balance bars to counter balance the vibrating mass of the flow tube. In order to maintain a dynamic balance over a range of material densities, the ratio of the flow tube vibration amplitude to the balance bar vibrational amplitude changes as material density changes. As the material density increases, the flow tube vibration amplitude decreases and the balance bar vibration amplitude increases so as to maintain equality in the momentum of the two vibrating members. The change in amplitude ratio between the flow tube and the balance bar changes the phase difference between the two pickoff signals in a manner that is best understood by using a vector diagram to predict the output of each pickoff. The net output signal out of the pickoff is a result of the vector addition of the phase shifted velocity of the flow tube and the oppositely phase shifted velocity of the balance bar. As the amplitude ratio changes with increasing material density, the length of the flow tube velocity vector decreases and the length of the balance bar velocity vector increases. The sum of these two vectors, which is proportional to the pickoff output decreases in-phase shift as the length and thus importance of the oppositely phase shifted balance bar grows. This decrease in-phase shift of the pickoff output results in a decrease in meter sensitivity with material density.
It is known to decrease flow sensitivity with the increase in material density. In the prior art method the balance bar has its stiffness reduced in a region on either side of the driver. The stiffness reduction causes the distortion of the balance bar in response to the Coriolis deformation of the flow tube to be greatly increased. It also causes the resonant frequency of the second bending mode of the balance bar to be reduced so that it is closer to, but still above, the drive frequency. (The second bending mode has the same deformed shape as the balance bar takes in response to the Coriolis deformation of the flow tube.) By properly sizing the frequency separation between the drive frequency and the resonant frequency of the second bending mode, a change in flow sensitivity with density cancels the change in flow sensitivity caused by the change in amplitude ratio with density. The problem with this method, however, is that the balance bar deformation due to the flow tube Coriolis deformation is much larger. Because the balance bar deformation results in a phase shift that is opposite to the phase shift caused by flow, a greater reduction in flow sensitivity results.
It is also known to make flow sensitivity independent of material density by using the same method of removing stiffness from the balance bar, but by removing enough stiffness to move the balance bar second bending mode resonant frequency below the drive frequency. This causes the Coriolis like deformation of the balance bar to shift phase by 180 degrees so that the phase associated with the deformation adds to the phase of the flow tube. Unfortunately, in practice it is extremely difficult to lower the second bending resonant frequency enough to give a sensitivity independent of material density.
A further disadvantage of the single straight tube Coriolis flowmeter is that the Coriolis force applied to the flow tube is unbalanced and results in flowmeter vibration and in flowmeter mounting sensitivity. The Coriolis force associated with mass flow is proportional to the flow rate. The Coriolis force on the inlet and outlet halves of the flow tube are equal but opposite in sign. This results in a rocking couple or torque on the flow tube. In dual tube meters, the rocking couple on the two tubes are in opposite directions and cancel each other. In single tube meters the balance bar experiences no Coriolis force so the rocking couple on the flow tube remains unopposed. This causes the flowmeter to shake an increased amount with increased flow rate. The shaking is at the meter drive frequency so that tube accelerations caused by the meter shaking are indistinguishable from Coriolis acceleration of the material. Furthermore, the meter shaking acceleration is added or subtracted from the Coriolis acceleration resulting in a measurement error. Meter mounting stiffness compounds the problem because the amount of meter shaking is inversely proportional to the mounting stiffness.
In summary, prior art single straight tube Coriolis flowmeters have reduced accuracy which the present invention addresses. These problems are low flow sensitivity, a flow sensitivity that changes with material density, and meter shaking due to unbalanced torque resulting from the Coriolis forces.
The above and other problems are solved and an advance in the art is achieved by the present invention in accordance with which a method and apparatus for a single straight tube Coriolis flowmeter is provided having a balance bar that includes an additional dynamic structure, termed a balance bar resonator. The resonator performs three functions that enhance the accuracy of output information generated by the flowmeter.
The first such function is that the balance bar resonator generates torques that are applied via the balance bar to the flow tube to counteract the torques applied to the flow tube by the Coriolis forces. This reduces the shaking of the flowmeter caused by the applied Coriolis forces. The second such function performed by the balance bar resonator is that the application of resonator torques to the balance bar reduces the amplitude of the Coriolis like deflections imparted to the balance bar by the Coriolis deflections of the flow tube. The balance bar resonator, is affixed to the longitudinal center of the balance bar to cancel in-phase Coriolis like deflections induced in the balance bar by the Coriolis deflections of the flow tube. These flow tube deflections generate in-phase Coriolis like deflections in the balance bar which, in turn, decrease flowmeter sensitivity. The reduction of the amplitude of these Coriolis like deflections by the balance bar resonator thus enhances flowmeter sensitivity. The third such function performed by the torques is to provide for a flat flowmeter calibration factor over a range of material densities as subsequently discussed.
The balance bar resonator consists of a relatively rigid bar oriented parallel to and beside the balance bar. The balance bar resonator bar may have weights affixed to its ends (if necessary) to reduce the resonant frequency of the working mode of the balance bar resonator. The balance bar resonator is coupled at its center to the center of the balance bar by a member called a strut. The balance bar resonator and the strut lie in the plane of vibration of the flow tube and balance bar in their drive mode. The working mode of the balance bar resonator is a rotational mode of vibration in which the rigid resonator bar rotates in the drive plane as the strut bends. The working mode of the balance bar resonator has a resonant frequency that is spaced apart and lower than the drive frequency.
The drive vibrations do not excite the working mode of the balance bar resonator because the longitudinal center of the balance bar, to which the strut is affixed, experiences no rotation, only translation. The strut translation with the balance bar causes balance bar resonator translation but does not excite working mode rotational vibrations in the balance bar resonator because the balance bar resonator bar is rigid and symmetrical about the strut. It takes the Coriolis like deflections of the balance bar to excite the working mode vibrations in the balance bar resonator. These deflections cause rotation of the balance bar center and bending of the balance bar resonator strut. Because the resonant frequency of the working mode of the balance bar resonator is lower than the frequency of the application of the Coriolis like deflections (the drive frequency), the balance bar resonator vibrates out of phase with the Coriolis like deflections of the balance bar. The bending of the strut thus applies a torque to the center of the balance bar that tends to decrease the rotation of the center of the balance bar and the amplitude of the Coriolis like deflections of the balance bar. The balance bar resonator works like a dynamic balancer of the balance bar in the rotation mode of the balance bar resonator. The balance bar resonator is also like a dynamic balancer in that the degree to which it reduces the Coriolis like deflection of the balance bar is inversely proportional to the separation of the drive frequency from the resonant frequency of the working mode. A very close spacing results in nearly complete cancellation of the Coriolis like deflection while a larger separation results in a lesser degree of cancellation. Since the Coriolis like deflections of the balance bar results in a decrease in meter sensitivity to flow, the cancellation of the deflection by the balance bar resonator increases the meter flow sensitivity.
The change in the degree of cancellation with frequency separation can be used to eliminate the change in meter flow sensitivity with material density. It has been previously shown how the change in flow tube/balance bar amplitude ratio causes the meter sensitivity to decrease with increasing material density. It has also been shown how the balance bar resonator reduces the Coriolis like deflections of the balance bar to a degree that is inversely proportional to the frequency separation between the drive frequency and the resonant frequency of the working mode. The balance bar resonator can be used to make the flow sensitivity independent of material density by using the frequency separation property along with the fact that the resonant drive frequency of the meter decreases as the material density increases.
When the material density increases, the drive frequency decreases. Since the resonant frequency of the working mode of the balance bar resonator is below the drive frequency and since it is independent of material density, the material density increase causes a decrease in the frequency separation between the drive frequency and the resonant frequency of the working mode. The decrease in frequency separation causes an increase in the amplitude of the balance bar resonator and a decrease in the Coriolis like deflections of the balance bar. This decrease causes an increase in sensitivity of the meter with material density. This increase in meter sensitivity with increasing density can be made to just cancel the decrease in meter sensitivity with density caused by the changing amplitude ratio. The increase in sensitivity with density caused by the balance bar resonator is greatest when the drive frequency with the high density material is equal to or slightly greater than the resonant frequency of the working mode. Increasing the separation with the high density material results in a lesser increase in sensitivity with density. Thus the increase in sensitivity due to the balance bar resonator can be made to cancel the decrease in sensitivity by the proper initial spacing between the drive frequency and the resonant frequency of the balance bar resonator.
As discussed, the balance bar resonator applies a torque to the balance bar to optimize flow sensitivity or make the flow sensitivity independent of changes in material density. This torque is proportional to and opposite to the Coriolis torque that is applied to the flow tube by the material flow. Even though the balance bar resonator torque is applied to the balance bar and the material torque is applied to the flow tube, both are ultimately applied to the meter case and flanges. The balance bar resonator torque thus reduces the net torque on the case and results in less meter shaking and less of the error associated meter with meter shaking.
Aspects of the invention comprise a method of operating and apparatus defining a Coriolis flowmeter adapted to receive a material flow and having:
a flow tube and a balance bar oriented substantially parallel to said flow tube;
brace bar means coupling end portions of said balance bar to said flow tube;
balance bar resonator means coupled to said balance bar;
a driver that vibrates said flow tube and balance bar out of phase with respect to each other in a drive mode having a frequency substantially equal to the resonant frequency of said material filled flow tube and said balance bar;
said material flow applies periodic Coriolis forces to said vibrating flow tube to generate periodic Coriolis deflections of said flow tube that are characterized by regions of deflection as well as nodes having no deflection;
said brace bar means is responsive to said Coriolis deflections of said flow tube to generate periodic Coriolis like deflections of said balance bar that are characterized by regions of deflection as well as nodes having no deflection;
said Coriolis like deflections of said balance bar are in-phase with and have the same number of nodes as said periodic Coriolis deflections of said flow tube;
said Coriolis like deflections include a rotation of a longitudinal center portion of said balance bar;
said in-phase Coriolis like deflections of said balance bar excite said balance bar resonator means to vibrate in a rotational mode out of phase with respect to said rotation of said longitudinal center portion of said balance bar;
apparatus whereby the vibration of said balance bar resonator means in said rotational mode applies torque to said balance bar that increases the accuracy of output information generated by said Coriolis flowmeter;
pickoff means coupled to said flow tube that generate signals of increased accuracy representing a vibrational velocity of said flow tube with respect to a vibrational velocity of said balance bar; and
meter electronics that derives information regarding said material flow in response to said generation of said signals of increased accuracy.
A further aspect is apparatus that increases the accuracy of said output information of said Coriolis flowmeter comprises:
apparatus, including said balance bar resonator means, that applies torque from said balance bar resonator means to said balance bar to decrease the amplitude of said Coriolis like deflections of said balance bar;
apparatus that increases the relative velocity of said Coriolis deflections of said flow tube with respect to said in-phase Coriolis like deflections of said balance bar in response to said decrease of said amplitude of said in-phase Coriolis like deflections of said balance bar; and
apparatus which increases the flow sensitivity of said Coriolis flowmeter in response to said increase in said relative velocity of Coriolis deflections of said flow tube with respect to said in-phase Coriolis like deflections of said balance bar.
A further aspect is that said Coriolis forces apply torque to said flow tube that causes shaking of said Coriolis flowmeter; said apparatus that increases the accuracy of said output information of said Coriolis flowmeter comprises:
apparatus that extends said torque applied by said balance bar resonator means to said balance bar and via said balance bar to said flow tube to reduce the torque applied by said flow tube to meter mounts of said Coriolis flowmeter;
said reduction of said torque applied by said flow tube to said meter mounts is effective to reduce the shaking of said Coriolis flowmeter.
A further aspect is that said apparatus that increases the accuracy of said output information of said Coriolis flowmeter comprises:
apparatus that detects a change in the resonant frequency of said vibrating flow tube and said balance bar resulting from a change in the density of said flowing material;
apparatus that changes the vibrational amplitude ratio of said flow tube and said balance bar in response to said detection of said change in material density;
apparatus that changes the material flow sensitivity of said Coriolis flowmeter in a first direction in response to said change in vibrational amplitude ratio:
apparatus that alters the vibrational amplitude of said balance bar resonator to change said material flow sensitivity in a second direction in response to said change in said resonant frequency;
said changes in flow sensitivity in said first direction and in said second direction are effective to achieve a flow calibration factor of said Coriolis flow meter.
A further aspect is that said apparatus that induces said in-phase Coriolis like deflections in said balance bar includes apparatus that extends forces indicative of said periodic Coriolis deflections from said flow tube through said brace bar means to said balance bar to induce said in-phase Coriolis like deflections in said balance bar.
A further aspect includes apparatus that couples said balance bar resonator means to said longitudinal center portion of said balance bar.
A further aspect is that said balance bar resonator means comprises:
an elongated bar substantially parallel to said balance bar at a rest state of said Coriolis flowmeter;
a stub coupling said elongated bar to the longitudinal center portion of said balance bar;
the vibration of said balance bar resonator means with respect to said longitudinal center portion of said balance bar applies torque to said balance bar.
A further aspect is that said applied axial torque from said balance bar resonator reduces the amplitude of said in-phase Coriolis like deflections of said balance bar to increase the flow sensitivity of said Coriolis flowmeter.
A further aspect is that said applied torque of said balance bar resonator is extended from said balance bar via brace bars to said flow tube to reduce the shaking of said Coriolis flowmeter.
A further aspect-is that said resonator bar includes mass.
A further aspect is that said mass comprises mass element affixed to ends of said resonator bar.
A further aspect is that said balance bar resonator means comprises said stub which is couples said elongated bar to said longitudinal center of said balance bar on a bottom surface of said balance bar.
A further aspect is that said balance bar resonator means comprises a first and a second balance bar resonator each comprising a and an elongated bar;
said stub of said first balance bar resonator is coupled to said longitudinal center of said balance bar on a first side of said balance bar and said stub of said second balance bar resonator is coupled to said longitudinal center of said balance bar on a second side of said balance bar.
A further aspect is that said apparatus that induces said in-phase Coriolis like deflection in said balance bar includes:
apparatus that flexes ends of said flow tube in response to said periodic Coriolis deflections to flex a first end of a brace bar means; and
apparatus that flexes a second end of said brace bar in response to said flexing of said first end to induce said in-phase Coriolis like deflections in said balance bar.
Yet another aspect is a method of operating a Coriolis flowmeter adapted to receive a material flow and having a flow tube and a balance bar that is oriented substantially parallel to said flow tube; said Coriolis flowmeter has brace bar means coupling said balance bar to said flow tube and further has a balance bar resonator means coupled to a longitudinal center portion of said balance bar; said method comprising the steps of:
flowing material through said flow tube;
vibrating said flow tube and balance bar out of phase with respect to each other in a drive mode having a drive frequency substantially equal to the resonant frequency of said material filled flow tube and said balance bar;
said flowing material applies periodic Coriolis forces to said vibrating flow tube to generate periodic Coriolis deflections of said balance bar that are characterized by regions of deflection as well as nodes having no deflection;
inducing in-phase Coriolis like deflections in said balance bar at said drive frequency in response to said Coriolis deflections of said flow tube;
said Coriolis like deflections include a rotation of said longitudinal center portion of said balance bar;
said Coriolis like deflections being in-phase with and having the same number of nodes as said periodic Coriolis deflections of said flow tube;
said Coriolis like deflections of said balance bar excite said balance bar resonator means to vibrate in a rotational mode out of phase with respect to said rotation of longitudinal center portion of said balance bar;
the rotational mode vibration of said balance bar resonator means applies torque to said balance bar to increase the accuracy of output information generated by said Coriolis flowmeter;
operating pick offs that generate signals of increased accuracy representing a vibrational velocity of said flow tube with respect to a vibrational velocity of said balance bar; and
deriving information of increased accuracy regarding said material flow in response to said generation of said signals of increased accuracy.